(1) Field of the Invention
The present invention relates to screw elements for multiscrew extruders with pairs of co-rotating and fully wiping screws, to the use of these screw elements in multiscrew extruders and to a method of generating these screw elements.
(2) Description of Related Art
Co-rotating twin- or multiscrew extruders whose rotors fully wipe each other have been known for a long time (see, for example, German Patent No. 862,668). Screw extruders based on the principle of fully wiping profiles have been used for many diverse applications in the field of polymer production and processing. This is mainly due to the fact that polymer melts adhere to surfaces and are degraded over time at the processing temperatures commonly employed. This is prevented by the self-cleaning effect of fully wiping screws. Rules for generating fully wiping screw profiles are described for example in Klemens Kohlgrüber: Der gleichläufige Doppelschneckenextruder (“The co-rotating twin-screw extruder”), Publishers: Hanser Verlag, Munich, 2007, pp. 96 et seq,) (“Kohlgrüber”) (The abbreviations, symbols and indices written in normal script in the figures are written in italics in the description.). This reference describes the construction of one-, two- and three-flight profiles.
Those skilled in the art are aware of the fact that in the region of the screw tips a particularly large amount of energy is dissipated in the melt, thus leading to considerable local overheating in the product. This is described for example in Kohlgrüber on page 160 et seq. of Kohlgrüber. This local overheating can lead to damage to the product by, for example, producing changes in its smell, color, chemical composition or molecular weight or to the formation of inhomogeneities, such as gelled particles or specks. A large tip angle is particular damaging in this regard.
In twin-screw extruders energy is introduced in the form of highly valuable electrical energy and it is therefore desirable, for cost-related and environmental reasons, to reduce the energy input. In addition, a high input of energy leads to high product temperatures, which can in turn produce disadvantages with regard to quality. In addition, a high input of energy in many cases reduces the possible throughput and thus also the cost-effectiveness of twin-screw extruders.
The input of energy in twin-screw extruders is determined by the process parameters of throughput and speed of rotation, by the material properties of the product and by the geometry of the screws employed. Modern twin-screw extruders consist of a modular system in which various screw elements can be mounted onto a central shaft. Using such a system those skilled in the art can adapt a twin-screw extruder to suit the respective processing task. Today screw elements with two- and three-flight profiles are usually employed, since one-flight screw profiles have an excessively high energy input due to their large tip angle.
According to the prior art (see, for example, page 101 of Kohlgrüber), the geometry of fully wiping screw elements is determined by using the independent parameters of flight number Z, centre distance A and barrel diameter (i.e. which corresponds to the diameter DE of the fully wiping contour). The flight number is the number of arcs of each element which wipe the outer wall. The angle of such an arc in relation to the centre of rotation is referred to as the tip angle KW0. In the region of the tip angle, the outer radius of the profile is the same as the barrel radius. According to the prior art, KW0 is not an adjustable parameter which can be modified to suit the problem at hand, but is given by the following equation 1:
                              KW          ⁢                                          ⁢          0                =                              π            Z                    -                      2            ⁢                                                  ⁢                          arccos              ⁡                              (                                  A                  DE                                )                                                                        (                  Eq          .                                          ⁢          1                )            wherein KW0 is the tip angle of the fully wiping profile in terms of radian measurement and π is pi (π≈3.14159). The sum of the tip angles of both elements of a tightly intermeshing pair of elements SKW0 is therefore as follows:
                              SKW          ⁢                                          ⁢          0                =                              2            ⁢            π                    -                      4            ⁢                                                  ⁢            Z            ⁢                                                  ⁢                          arccos              ⁡                              (                                  A                  DE                                )                                                                        (                  Eq          .                                          ⁢          2                )            
If regions of a twin-screw extruder are only partially filled with melt during operation, for example in a degassing zone or in the buffer region of a pressure build-up zone, the melt rotates downstream of the tips Kohlgrüber. Each screw profile has one flank which “pushes” the melt and one flank which “pulls” the melt. The screw rotates in such a manner that the “pushing” flank is arranged on the downstream side of the tip and the pulling flank on its upstream side. In the partially filled state the melt rotates downstream of the “pushing” flank. The dissipation of energy and processing efficiency, for example for degassing operations, in this rotating melt depends not only on the tip angle and the clearances but also on the geometry of the melt channel downstream of the “pushing” flank. The prior art does not provide any possibility of adapting this geometry to suit the problem to be solved.
During operation, the screws of multiscrew extruders are usually mounted in the gearbox at the drive end, which is at the same time the product feed end. At the product ejection end the screws are mounted in the molten product, since external mounting would be a hindrance in the product ejection zone. Before a multiscrew extruder is charged with product during a start-up process, the screws rotate without lubrication of their tips directly on the barrel material. This can lead to abrasion, damage to the screw and the barrel and contamination of the product. In order to avoid excessive wear of the tips, a certain minimum tip angle is required. It would therefore be desirable to be able to freely select this tip angle.
Twin-screw extruders can also be subject to wear, which can occur in the melting zone in the case of pure polymers. Products which are filled with solid filling and reinforcing materials such as for example talcum, calcium carbonate or in particular glass fibres, produce a particularly high degree of wear. Corrosive attack is, for example, also possible when the product contains acids or undergoes cleavage. Such abrasion and corrosive attacks have a particularly detrimental effect on the crests at the edges of a profile tip where it is, for example, possible for material to be worn away or for crumbling to occur. Such changes to the profile tip have a crucial effect on the efficiency of multiscrew extruders and this is undesirable. Rounded crests would be considerably less susceptible to such effects but cannot be used according to the prior art without losing the self-cleaning effect of the screws.
In the light of the prior art, the problem therefore arose of providing tightly intermeshing screw elements for multiscrew extruders which are not subject to the abovementioned restrictions of the screw elements according to the prior art. The problem was to provide screw elements in which the energy input is reduced. The problem was also to provide screw elements in which the geometry of the pushing and the pulling flanks can be designed in such a manner in relation to the problem to be solved that optimum processing of the product can be carried out in a multiscrew extruder.
Surprisingly, screw elements have been obtained which have a reduced tip angle compared to the prior art and which solve the abovementioned problems.